The presently disclosed embodiments relate generally to layers that are useful in imaging apparatus members and components, for use in electrostatographic, including digital, apparatuses. More particularly, the embodiments pertain to an improved electrostatographic imaging member having a specific overcoat formulation that provides excellent mechanical properties while reducing the amount of formaldehyde released or generated and processes for making the same. In embodiments, the photoreceptor comprises an overcoat having a formaldehyde scavenger therein. In embodiments, the formaldehyde scavenger is selected from the group consisting of ethylene urea, dimethylol ethylene urea and mixtures thereof.
Electrophotographic imaging members, e.g., photoreceptors, photoconductors, imaging members, and the like, typically include a photoconductive layer formed on an electrically conductive substrate. The photoconductive layer is an insulator in the substantial absence of light so that electric charges are retained on its surface. Upon exposure to light, charge is generated by the photoactive pigment, and under applied field charge moves through the photoreceptor and the charge is dissipated.
In electrophotography, also known as xerography, electrophotographic imaging or electrostatographic imaging, the surface of an electrophotographic plate, drum, belt or the like (imaging member or photoreceptor) containing a photoconductive insulating layer on a conductive layer is first uniformly electrostatically charged. The imaging member is then exposed to a pattern of activating electromagnetic radiation, such as light. Charge generated by the photoactive pigment move under the force of the applied field. The movement of the charge through the photoreceptor selectively dissipates the charge on the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image. This electrostatic latent image may then be developed to form a visible image by depositing oppositely charged particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the imaging member directly or indirectly (such as by a transfer or other member) to a print substrate, such as transparency or paper. The imaging process may be repeated many times with reusable imaging members.
An electrophotographic imaging member may be provided in a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. In addition, the imaging member may be layered. These layers can be in any order, and sometimes can be combined in a single or mixed layer.
Typical multilayered photoreceptors or imaging members have at least two layers, and may include a substrate, a conductive layer, an optional charge blocking layer, an optional adhesive layer, a photogenerating layer (sometimes referred to as a “charge generation layer,” “charge generating layer,” or “charge generator layer”), a charge transport layer, an optional overcoating layer and, in some belt embodiments, an anticurl backing layer. In the multilayer configuration, the active layers of the photoreceptor are the charge generation layer (CGL) and the charge transport layer (CTL). Enhancement of charge transport across these layers provides better photoreceptor performance.
The term “photoreceptor” or “photoconductor” is generally used interchangeably with the terms “imaging member.” The term “electrostatographic” includes “electrophotographic” and “xerographic.” The terms “charge transport molecule” are generally used interchangeably with the terms “hole transport molecule.”
One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990, which describes a photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer (CTL). Generally, where the two electrically operative layers are supported on a conductive layer, the photoconductive layer is sandwiched between a contiguous CTL and the supporting conductive layer. Alternatively, the CTL may be sandwiched between the supporting electrode and a photoconductive layer. Photosensitive members having at least two electrically operative layers, as disclosed above, provide excellent electrostatic latent images when charged in the dark with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely divided electroscopic marking particles. The resulting toner image is usually transferred to a suitable receiving member such as paper or to an intermediate transfer member which thereafter transfers the image to a member such as paper.
In the case where the charge-generating layer (CGL) is sandwiched between the CTL and the electrically conducting layer, the outer surface of the CTL is charged negatively and the conductive layer is charged positively. The CGL then should be capable of generating electron hole pair when exposed image wise and inject only the holes through the CTL. In the alternate case when the CTL is sandwiched between the CGL and the conductive layer, the outer surface of CGL layer is charged positively while conductive layer is charged negatively and the holes are injected through from the CGL to the CTL. The CTL should be able to transport the holes with as little trapping of charge as possible. In flexible web like photoreceptor the charge conductive layer may be a thin coating of metal on a thin layer of thermoplastic resin.
In a typical machine design, a drum photoreceptor is coated with one or more coatings applied by well known techniques such as dip coating or spray coating. Dip coating of drums usually involves immersing of a cylindrical drum while the axis of the drum is maintained in a vertical alignment during the entire coating and subsequent drying operation. Because of the vertical alignment of the drum axis during the coating operation, the applied coatings tend to be thicker at the lower end of the drum relative to the upper end of the drum due to the influence of gravity on the flow of the coating material. Coatings applied by spray coating can also be uneven, e.g., orange peel effect. Coatings that have an uneven thickness do not have uniform electrical properties at different locations of the coating. Under a normal machine imaging function condition, the photoreceptor is subjected to physical/mechanical/electrical/chemical species actions against the layers due to machine subsystems interactions. These machine subsystems interactions contribute to surface contamination, scratching, abrasion and rapid surface wear problems.
As electrophotography advances, the complex, highly sophisticated duplicating systems need to operate at very high speeds which places stringent requirements on imaging members and may reduce imaging member longevity. Thus, there is a continued need for achieving increased life span of photoconductive imaging members while maintaining good mechanical properties. In addition, although present photoreceptors provide excellent mechanical properties such as abrasion resistance, crack resistance and wear resistance, known crosslinking agents contain and/or generate formaldehyde in small quantities. Small quantities of formaldehyde are objectionable in the manufacturing plant due to the necessity of protective gear to avoid exposure, for example, exposure limits are less than 0.5 ppm. If transferred/coated in the open atmosphere of the pilot plant, levels can approach 20 ppm. The addition equipment to protect the plant personnel can be expensive. Therefore, it is desired to provide a photoreceptor that reduces and minimizes formaldehyde exposure.